Oddly, new battery trays are available...aftermarket of course. I have one in my truck. The factory design was really awful when you consider what road salt does to sheetmetal.
They're available from LMC for sure, and I think I've seen 'em on Rock Auto and Ebay as well. I bought the whole works from LMC with the hold down "J" bolts and nuts, and the plastic brace across the top of the battery. I believe they even sell the little triangle bracket that attaches to the core support, although that one is more prone to getting removed and lost as opposed to rusting away.
After market battery trays:
https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=Ford+F150+battery+tray+1986&_sacat=0&LH_TitleDesc=0&_osacat=0&_odkw=Ford+F150+battery+tray+NOS+1986